An evaluation of the hydrogen explosion hazard during severe accidents is made using recent experimental and theoretical results. It is possible to estimate geometric conditions under which detonations can propagate for a given mixture composition on the basis of detonation cell size data. Multidimensional modelling is suggested to improve the description of detonation propagation conditions. The conditions of spontaneous detonation or strong explosion processes' initiation including a deflagration-to-detonation transition (DDT) and a detonation self-initiation mode in non-uniform mixtures (the shock wave amplification by coherent energy release mechanism) are analysed. Detonation self-initiation is considered as the part of the DDT process that determines the minimum detonation formation distance. The evaluation of this distance in the self-initiation mode is suggested for various mixture compositions on the basis of the values of minimum initiation energy or detonation cell width. This approach gives the possibility of hydrogen explosion hazard estimation in spite of the many uncertainties and important unresolved questions about the DDT and spontaneous detonation.